In the following description, terminology is used in the broadest sense. Particularly:                the term “sea” is used to indicate any suitable body of water, whether by name it would be indicated as sea or ocean, or even river or canal;        the term “ship” is used to indicate any suitable towing device travelling in the sea or on the sea surface, irrespective of its propulsion system, irrespective of its control system, and irrespective of the presence of a crew.        
In the art of reflection seismology, acoustic pulses are generated at the upper regions of a sea (depth of about 5 to about 100 meters), and reflected acoustic signals are measured and analysed. This technology is useful, for instance, for mapping the sea floor and for exploring for oil and gas, in which case the structure below the floor surface is to be mapped.
The acoustic waves travel in the water as pressure waves, and are detected by pressure sensors. In a practical setup, a large plurality of sensors is arranged along the length of a cable of several kilometers long, with a mutual distance in the order of a few meters. The cable is towed in the water behind a ship; such towed cable is also indicated as “streamer”. Measuring signals from the sensors travel along the streamer to a processing apparatus, which may or may not be located aboard the ship. In practice, the ship will be towing a plurality of such streamers parallel to each other, at a mutual distance in the order of about 50 meters. So all in all, a measuring array of many thousands of pressure sensors will be in operation.
In a typical prior art example, the pressure sensor is implemented as a piezo element, which comprises a piezo crystal. Pressure variations cause the piezo crystal to contract or expand, which in turn causes the piezo crystal to generate electrical signals. In such case, for transporting these electrical signals, a streamer needs to contain electrically conductive lines, which are typically made of copper, but which may alternatively be made of aluminium. In order to keep signal losses low, the conductive lines must be relatively thick. Alternatively, or additionally, such streamers need to include data acquisition units for combining and multiplexing or digitising the sensor signals. The same applies to other types of streamers, where the pressure sensors generate electrical signals.
It has already been proposed to replace the electrical signals by optical signals. This would allow the copper signal lines to be replaced by optical fibres. Instead of active sensors, which themselves generate optical signals, passive sensors have been proposed. With the phrase “passive” in this context is meant that an optical property of such sensor varies in response to variations in an ambient parameter, which optical property can be measured by interrogating the sensor with light. A passive optical element that has proven itself in this respect is a so-called Fibre Bragg Grating (FBG) reflector.
An FBG reflector consists of an optical fibre wherein, at some location, a series of material modifications is arranged lengthwise in the fibre. Normally, the optical properties of an optical fibre are constant along the length, which optical properties include the refractive index. Such material modification, however, has a slightly different refractive index. A plurality of such material modifications, at mutually the same distance, behaves as a grating, which typically is reflective for a small wavelength band. If a light pulse is made to enter the fibre, substantially all wavelengths will pass the grating location but light within said small wavelength band will be reflected. At the input end of the fibre, a reflected light pulse will be received, of which the wavelength is indicative for the mutual distance between the successive material modifications.
Such FBG reflector sensor is typically sensitive to (local) strain. Variations in strain cause variations in length of the fibre, including variations in distance between the successive material modifications of the Bragg grating. This, in turn, translates to variations in the wavelength of the reflected light.
It is noted that FBG reflectors are known per se, and that the use of FBG reflectors in streamers is known per se. Reference in this respect is made to, for instance, US patent applications 2011/0096624 and 2012/0069703. Since furthermore the present invention is not directed to providing an improved optical fibre or an improved FBG, while the present invention can be implemented using optical fibres with FBG reflectors of the same type as currently are being deployed, a more detailed explanation of design and manufacture of optical fibres with FBG reflectors is omitted here.
The acoustic waves to be sensed are pressure waves in the sea water. Since the FBG reflectors are mainly sensitive to longitudinal strain variations, a pressure sensor having an FBG reflector as sensitive element needs to have means for translating ambient pressure variations to fibre strain variations.